A conventional type of color video device with a cathode-ray tube comprises a demagnetization coil with associated circuitry which permits the demagnetization of the cathode-ray tube. The demagnetization coil is fixed to the cathode-ray tube and is fed by AC power as the video device is switched on through a thermistor (PTC) with a positive temperature coefficient. The current passing through the coil heats the thermistor, increasing the resistance thereof, thus exponentially limiting the current itself. Additional resistances, thermally coupled to the thermistor, are needed to maintain the thermistor at a desired temperature during the normal operation of the video device.
Unfortunately, such a demagnetization circuit has some disadvantages. One of the main disadvantages, due to the existing environmental requirements relating to energy savings, resides in that both during the normal operation and during the operation at reduced power (stand-by) a non-negligible amount of power is dissipated in heat by the thermistor itself and by the resistances coupled thereto. At present, in fact, TV-sets are normally left in a stand-by state, a condition where the main loads, such as the audio amplifier and the vertical and horizontal deflection circuits, are not connected. However, the power supply is still connected to the AC power and keeps supplying a non-negligible amount of power.
The main reasons for the above mentioned power consumption in the stand-by state may be listed as follows:
power consumed by the power supply to supply energy to the microchip and to the infrared receiver connected thereto, which must guarantee the switch-on of the TV-set, such power being about 5 Watts, and makes use of a switching power supply (Switched Mode Power Supply--SMPS) that foresees an operational stand-by state named "Burst-mode" at reduced consumption rate; and PA1 power consumed by the thermistor (PTC) connected in series with the demagnetization coil and by the resistances coupled thereto, averaging about 1.5 Watts.
This last item is a mere loss, as it is not needed for keeping the stand-by condition itself.
The demagnetization function provided by the PTC thermistor is exclusively active at the first switch-on of the TV-set and ends when the thermistor has heated up and its internal resistance has reached the maximum value. Accordingly, at any successive switch-on from the stand-by state, the cathode-ray tube is no longer being demagnetized as the thermistor is already warm, and, therefore, at its maximum value of internal resistance.
Unfortunately, this may represent a serious problem if the TV-set is left in the stand-by state for a long time or if a sudden change in the magnetic field takes place inside or outside the TV-set itself (due to the movement or approaching of magnetic material). By changing from the stand-by state to the switched-on state, the TV tube will show colored spots that modify the regular chromaticity of the image visualized. Only after the complete switch-off of the TV-set and a successive switch-on again, will the TV-tube again be efficient, having been demagnetized.